An example of data transfer systems presently in use is the data transfer system of a facsimile equipment. The data transfer system of a facsimile uses a common bus through which graphic information is transferred between various circuit components of the system including a central processor unit. In such a data transfer system, it has been an ordinary practice to transfer graphic information on the common bus for each of the lines scanned from a subject copy or each of the lines to be reproduced. When white lines occur successively, the graphic information is encoded faster than graphic information is to be coded in other conditions and, for this reason, a large quantity of graphic information must be transferred through the common bus for a relatively short period of time. The larger the proportion of the share of the graphic information signals which occupy the common bus, the smaller the proportion of the share of the central processing unit which is allowed to use the common bus. This results in significant reduction in the data transfer efficiency of the common bus.
Where the data transfer system is arranged so that some lines are to be skipped over as implemented in the data transfer systems of some facsimile equipments, the number of the lines to be skipped is expressed by a sequence of a predetermined number of bits. To provide matching between the readout or record rate and the data transfer rate, some invalid bits are usually added to the sequence of these bits when coded data is to be transferred on the common bus. When a relatively small number of lines are to be skipped over, an increased number of invalid bits are added to the sequence of the bits expressing the number of the lines to be skipped. When an increased number of lines are to be skipped, on the other hand, the sequence of the bits to express the number of the lines is broken down to two or more component sequences each containing invalid bits. A considerably large number of invalid bits must be in any event contained in the bits to be transferred on the common bus even though the data is to be transferred in a compressed state. This is also responsible for the reduced data transfer efficiency of the common bus.